The thin filaments of smooth muscle have an actin and tropomyosin composition and structure similar to that in skeletal muscle. However, the cellular content of actin and tropomyosin in smooth muscle is about twice that of striated muscle. Smooth muscle lacks troponin and nebulin but contains two proteins not found in striated muscle: caldesmon and calponin. The precise roles of these proteins are unknown, but they do not appear to be fundamental to cross-bridge cycling. It has been suggested that both calponin and caldesmon may regulate the contractility of smooth muscle. (in part by inhibiting actomyosin ATPase activity). Most of the myoplasm is filled with thin filaments that are roughly aligned along the long axis of the cell. The myosin content of smooth muscle is only a fourth that of striated muscle. Small groups of three to five thick filaments are aligned and surrounded by many thin filaments. These groups of thick filaments with interdigitating thin filaments are connected to dense bodies or areas (
Fig. 14.4
Fig. 14.3A–B ) and represent the equivalent of the sarcomere. The contractile apparatus of adjacent cells is mechanically coupled by the links between membrane-dense areas.
The cytoskeleton in smooth muscle cells serves as an attachment point for the thin filaments and permits transmission of force to the ends of the cell. In contrast to skeletal muscle, the contractile apparatus in smooth muscle is not organized into myofibrils, and Z lines are lacking. The functional equivalents of the Z lines in smooth muscle cells •Fig. 14.4 Apparentorganizationofcell-to-cellcontacts,cytoskeleton,andmyofilamentsinsmoothmusclecells.Smallcontractileelementsfunctionallyequivalenttoasarcomereunderliethesimilaritiesinmechanicsbetweensmoothandskeletalmuscle.Linkagesconsistingofspecializedjunctionsorinterstitialfibrillarmaterialfunctionallycouplethecontractileapparatusofadjacentcells.Densebodies,thefunctionalequivalentofZlinesinstriatedmuscle,areinterconnectedbyintermediatefilaments.Themyofilamentsareorientedlargelyinparallelwiththelongitudinalaxisofthecell,thoughanobliqueorientationhasbeenobservedinsomearteries.
are ellipsoidal dense bodies in the myoplasm and dense areas that form bands along the sarcolemma (see
Figs. 14.3A–B 14.4 ). These structures serve as attachment points for the thin filaments and contain α-actinin, a protein also found in the Z lines of striated muscle. Intermediate filaments with diameters between those of thin filaments (7 nm) and thick filaments (15 nm) are prominent in smooth muscle. These filaments link the dense bodies and areas into a cytoskeletal network. The intermediate filaments consist of protein polymers of desmin or vimentin.
Control of Smooth Muscle Activity
The contractile activity of smooth muscle can be controlled by numerous factors, including hormones, autonomic nerves, pacemaker activity, and a variety of drugs. Like skeletal or cardiac muscle, contraction of smooth muscle is dependent on Ca++ , and the agents just listed induce smooth muscle contraction by increasing intracellular [Ca++]. However, in contrast to skeletal or cardiac muscle, action potentials in smooth muscle are highly variable and not always needed to initiate contraction. Moreover, several agents can increase intracellular [Ca++] and hence contract smooth muscle without changing the membrane potential.
Fig. 14.5 shows various types of action potentials in smooth muscle and the corresponding changes in force. An action potential in smooth muscle can be associated with a slow twitch-like response, and the twitch forces can summate during periods of repetitive action potentials (i.e., similar to tetany in skeletal muscle). Such a pattern of activity is characteristic of single-unit smooth muscle in many viscera.